As conventional transmitter circuits, for example, transmitter circuits (hereinafter, referred to as quadrature modulation circuits) that generate transmission signals by using a modulation mode such as a quadrature modulation have been known. The quadrature modulation circuits are well known and the description thereof is not given. Further, as conventional transmitter circuits each of which has a reduced size and operates with enhanced efficiency as compared to the quadrature modulation circuits, polar modulation circuits using a polar modulation mode have been known. In the polar modulation circuit, an inputted signal is separated into a phase component signal and an amplitude component signal, and amplification is performed to combine, by means of a saturated amplifier, a signal having a constant amplitude, which has been obtained by modulation using the phase component signal, with the amplitude component signal, thereby generating a transmission signal. In the polar modulation circuit, since an amplifier is operated in a saturated state, the transmission signal can be generated with a high power-efficiency in general.
Conventionally, a transmitter circuit has been suggested in which the polar modulation mode as described above is used for a high output, and the quadrature modulation mode is used for a low output, to obtain a high power-efficiency as a whole. For example, Patent Literature 1 discloses a transmitter circuit 600 as shown in FIG. 18. A configuration and an operation of the transmitter circuit will be described below.
An in-phase component signal (I signal) and a quadrature component signal (Q signal) which are signals for the quadrature mode are inputted to an interface section 602 from a baseband section 601. An Rθ transformation section 604 in the interface section 602 switches between the quadrature modulation mode and the polar modulation mode based on an AGC control signal from the baseband section 601. The Rθ transformation section 604 passes and outputs the I signal and the Q signal without performing signal processing, in the quadrature modulation mode, while performs a transformation process (Rθ transformation process) for transforming the I signal and the Q signal into an amplitude component signal and a phase component signal in the polar modulation mode. The Rθ transformation process is performed by extraction of phase information and envelop detection being performed by a limiter.
The I signal is inputted to a DAC 605 in the quadrature modulation mode, and the phase component signal is inputted to the DAC 605 in the polar modulation mode. Further, the Q signal is inputted to the DAC 606 in the quadrature modulation mode, and the amplitude component signal is inputted to the DAC 606 in the polar modulation mode. An output of the DAC 605 is inputted to a mixer 621 via a baseband filter 625.
A switch 607 operates so as to connect an output of the DAC 606 to a Q component baseband filter 608 of an RF-IC 603 in the quadrature modulation mode, and connect the output of the DAC 606 to an amplitude modulation circuit 609 in the polar modulation mode.
A switch 610 operates to connect a sum of the I signal and the Q signal to an AGC amplifier 611 in the quadrature modulation mode, and connect only the phase component signal to the AGC amplifier 611 in the polar modulation mode. A switch 612 operates to connect, in the quadrature modulation mode, an output of the AGC amplifier to an output buffer 613, and output the output of the AGC amplifier to a front-end section without passing the output through a power amplifier 614, while operates to connect, in the polar modulation mode, the output of the AGC amplifier to the power amplifier 614, to amplify the output.
In the quadrature modulation, the input signal is transformed into the I signal and the Q signal. An oscillation signal generated by an oscillator 620 is separated into two signals by a phase shifter 623. One of the signals is not subjected to phase shift, is outputted to a mixer 621, and is multiplied, in the mixer 621, by the I signal outputted from a baseband filter 625. The other of the signals is subjected to phase shift, and is thereafter outputted to a mixer 622, and is multiplied, in the mixer 622, by the Q signal outputted from a baseband filter 608. The signals obtained by multiplication by the I signal and multiplication by the Q signal, respectively, are then added by an adder 624, to obtain a modulated wave for the quadrature modulation mode. On the other hand, in the polar modulation, the input signal is transformed into the amplitude component signal and the phase component signal. The oscillation signal generated by the oscillator 620 is firstly multiplied by the phase component signal, to generate a phase-modulated signal. Thereafter, the phase-modulated signal is amplified and combined with the amplitude component signal by the power amplifier 614, to obtain a modulated wave for the polar modulation mode. Namely, the transmitter circuit switches the modulation mode such that the quadrature modulation is performed when a voltage level of a signal is lower than a predetermined value, and the polar modulation is performed when the voltage level of the signal is higher than the predetermined value. Thus, the conventional transmitter circuit uses the quadrature modulation mode and the polar modulation mode in combination, thereby realizing reduction of power consumption.